Hinged joint system

ABSTRACT

Methods, systems, and devices for replacement of a joint with a prosthetic system that replicates the natural kinematics of the joint is disclosed. A prosthetic system according to one embodiment includes a tibial component having a tibial plateau and a tibial stem portion, the tibial plateau having a top side and a bottom side, a tibial insert, with a bearing surface, adapted to be positioned on the top side of the tibial plateau, a femoral component having a base portion and a central housing, the femoral component having an axis of extension-flexion rotation, the base portion having a pair of condyles, an a mechanical linkage component linking the tibial component with the femoral component and with the tibial insert in between the tibial component and the femoral component, so that there is a center of contact between the condyles and the bearing surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/963,306, filed Aug. 9, 2013, now allowed, which is a continuation ofU.S. patent application Ser. No. 12/353,295, filed Jan. 14, 2009, nowU.S. Pat. No. 8,545,570, issued Oct. 1, 2013, which is a continuation ofU.S. patent application Ser. No. 10/499,047, filed Jun. 16, 2004, nowU.S. Pat. No. 7,572,292, issued Aug. 11, 2009, which is a national phaseof International Application No. PCT/US02/41221, filed Dec. 20, 2002,which claims priority from U.S. Provisional Application No. 60/342,350filed Dec. 21, 2001. The contents of the prior applications areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to prosthetic joints, and moreparticularly to a hinged joint that allows for the natural kinematics ofthe joint.

BACKGROUND OF THE INVENTION

In primary knee joint replacement surgery, a surgeon typically affixestwo prosthetic components to the patient's bone structure; a first tothe patient's femur and a second to the patient's tibia. Thesecomponents are typically known as the femoral component and the tibialcomponent respectively. In a typical primary knee joint replacementsurgery the ligaments and tendons are sufficiently intact to control themovement of the knee.

The femoral component is placed on a patient's distal femur afterappropriate resection of the femur. The femoral component is usuallymetallic, having a highly polished outer condylar articulating surface,which is commonly J-shaped.

A common type of tibial component uses a tray or plateau that generallyconforms to the patient's resected proximal tibia. The tibial componentalso usually includes a stem which extends at an angle to the plateau inorder to extend into a surgically formed opening in the patient'sintramedullary canal. The tibial component and tibial stem are bothusually metallic.

A plastic or polymeric (often ultra high molecular weight polyethyleneor UHMWPE) insert or bearing fits between the tray of the tibialcomponent and the femoral component. This tibial insert provides asurface against which the femoral component condylar portionarticulates—moves in gross motion corresponding generally to the motionof the femur relative to the tibia.

In some knee prostheses, the tibial insert also engages in motionrelative to the tibial tray. Such motion can be translational and-orrotational sliding motion relative to the tibial plateau. In other typesof knee prostheses with tibial inserts, the tibial inserts can engage inother types of motion relative to the tibial plateau and-or femoralcomponent.

Revision surgery is required when the primary prosthesis fails. In mostrevision cases additional stabilization and structure are necessary tocompensate for loss of bone and soft tissue. For example, the femoraland tibial components may be thicker to make up for the loss of bone.The femoral component may include a stem, which generally extends atabout six degrees from perpendicular from the base portion of thefemoral component in order to extend into a surgically formed opening inthe patient's intramedullary canal. In order to provide increasedstabilization, a box may be provided on the femoral component and amating post on the tibial component, creating what is called aconstrained knee replacement.

In some cases, the loss of soft tissue in the knee requires the use of alinked or hinged knee prosthesis. The three most common indications thata hinged knee is necessary are: (1) in an increasing number of revisioncases, the patient loses too much bone and soft tissue to use aconstrained knee; (2) an oncologist may be forced to resect a largeportion of a bone in order to remove a tumor; and (3) in traumaapplications, often the distal femur or proximal tibia has been crushedand must be replaced.

Early hinged knees were fixed, allowing no internal-external rotation.These early hinges had a history of loosening because their fixationcould not adequately handle applied forces. Rotating hinges decreasedthis failure because these rotating hinges minimized internal-externalrotational torque. Hinged knee systems provide a physical link of twocomponents with an axle, such that all medial-lateral andanterior-posterior stability is provided by the prosthesis. Thesesystems also address various degrees of bone loss. During normalarticulation, the pivot axis for the axle is fixed in theanterior-posterior and superior-inferior directions, so that when theknee is flexed or extended about the axle the center of contact betweenthe femoral and tibial components is fixed. This prevents roll-back.

A major concern with hinged knees is simulating the movement of anatural knee joint. The movement of a natural knee joint has threetranslations: anterior-posterior, medial-lateral, and inferior-superiorand three rotations: flexion-extension, internal-external, andabduction-adduction. The movements of the knee joint are determined bythe shape of the articulating surfaces of the tibia and femur and theorientation of the major ligaments of the knee joint, including theanterior and posterior cruciate ligaments and the medial and lateralcollateral ligaments as a four linkage system. Knee flexion-extensioninvolves a combination of rolling and sliding of the femur on the tibialplateau called femoral roll-back. In roll-back during flexion, thecenter of contact between the femur and the tibial plateau movesposteriorly, which allows increased ranges of flexion and increasedefficiency of the extensor mechanism.

Current hinged knees typically allow both hinge-over in theflexion-extension direction and internal-external rotation, but do so byflexing about a fixed pivot axis that eliminates roll-back. Some hingedknee designs, on the other hand, have hinge mechanisms that allowroll-back, but do not control roll-back. No known hinged knee systemsboth allow and control roll-back.

During pre-op planning the extent of bone and soft tissue damage is notalways discernable. Since surgical preference typically is to use theleast intrusive procedure, a revision with a constrained prosthesis, asopposed to a hinge knee, is preferred. If, during surgery, it becomesapparent that a hinge knee is necessary, it would be preferable for thehinge to be part of an integrated system so the surgeon can proceed withminimal interruptions. Current hinged systems are stand alone, so thatif the surgeon plans to use a constrained knee but realizes duringsurgery that the added constraint of a hinged knee is required, thesurgeon cannot switch to a hinged knee during the procedure. Rather, thesurgeon typically has to start another procedure resulting in longeroperating times and greater risk to the patient. Additionally, currenthinged knees require the surgeon to remove a large portion of thepatient's bone in order to allow proper implantation.

Current hinged knee systems require a considerable amount of assemblyduring surgery in order to ensure that the various components areproperly sized and connected. Such assembly takes time, is tedious andprone to error, and averts the surgeon's attention from more criticalmatters directly related to the health of the patient.

Thus, there is a current need for a hinged knee prosthesis that providesnatural kinematics without excessive bone removal. There is also a needfor a hinged knee system that is compatible with existing total kneereplacement systems. Finally, there is a need for a hinged knee systemthat requires less assembly during surgery.

SUMMARY OF THE INVENTION

Methods, systems, and devices for replacement of a joint with aprosthetic system that replicates the natural kinematics of the jointare disclosed. Methods, systems, and devices according to the inventionnot only allow, but also control, the roll-back and kinematics of theprosthesis, and thus the joint, and provide both natural biomechanicsand joint performance. Some existing hinged knee designs provide linkedarticulation in substitution for soft tissue deficiencies, but do so byflexing about a fixed pivot axis eliminating roll-back. The prior artthat allows movement of the axis of rotation or axle allows the axle tomove in the anterior and posterior directions, but does not control themovement. Some prior art discloses an axis of rotation near the centerof the femoral component and other prior art discloses an axis ofrotation in the rear portion of the femoral component. This prior artallows the femoral component and femur to move in the anterior andposterior directions relative to the tibia, but does not control themovement. The present invention controls roll-back through the operationof its linkage component. A prosthetic system according to oneembodiment of the invention includes a tibial component having a tibialplateau and a tibial stem portion, the tibial plateau having a top sideand a bottom side, a tibial insert, with a bearing surface, adapted tobe positioned on the top side of the tibial plateau, a femoral componenthaving a base portion, a central housing and a femoral stem portion, thefemoral component having an axis of extension-flexion rotation, the baseportion having a pair of condyles, a mechanical linkage componentlinking the tibial component with the femoral component and with thetibial insert in between the tibial component and the femoral component,so that there is a center of contact between the condyles and thebearing surface, the mechanical linkage component adapted to allow thecenter of contact to move posteriorly during flexion, provide for themovement of the axis of extension-flexion rotation in thesuperior-inferior direction, allow rotation of the tibial component, thetibial insert, and the femoral component about a superior-inferior axis,and offset the axis of extension-flexion rotation from thesuperior-inferior axis in order to provide and control the naturalkinematics of the knee joint.

A prosthetic system according to one embodiment of the inventionincludes a mechanical linkage component linking the tibial componentwith the femoral component and with the tibial insert in between thetibial component and the femoral component so that there is a center ofcontact between the condyles and the bearing surface, the mechanicallinkage component adapted to move in the superior-inferior directionsand restrained from movement in the anterior-posterior directions,wherein the center of contact between the condyles and bearing surfacemoves in the anterior-posterior direction as the femoral component movesthrough extension and flexion.

A prosthetic system according to one embodiment of the inventionincludes a tibial component having a tibial plateau and a tibial stemportion, the tibial plateau having a top side and a bottom side, a posthaving a proximal end and a distal end, the post adapted to project fromthe top side of the tibial plateau, a cap adapted to mount on theproximal end of the post, a tibial insert having an aperture, the tibialinsert adapted to be positioned on the top side of the tibial plateauwith the tibial insert aperture adapted to receive the post and the cap,a femoral component having a base portion and a central housing having afemoral stem portion, the base portion having a pair of condyles and twoposteriorly extending lobes, an axle adapted to connect to the lobes andextend between the lobes, and a link having an anterior end and aposterior end, the link adapted to be connected to the axle at theposterior link end and to receive the post and the cap at the anteriorlink end.

A method for replacing a joint with a prosthetic system according to oneembodiment of the invention includes resecting the proximal end of thepatient's tibia to expose the tibial intramedullary canal of the tibia,resecting the distal end of the patient's femur to expose the femoralintramedullary canal, connecting a tibial stem and a femoral stem to aprosthetic system, the prosthetic system having a mechanical linkagecomponent, inserting the tibial stem into the tibial intramedullarycanal, and inserting the femoral stem into the femoral intramedullarycanal. The method allows a surgeon to convert to the prosthetic systemfrom a primary or revision prosthesis with common bone cuts andinstrumentation. The prosthetic system allows the surgeon to select anappropriately sized tibial insert and a cap so that a pre-assembledfemoral component can be used, thereby significantly reducing the amountof surgical time devoted to assembly of the knee.

Another feature of the present invention is that a bioresorbable bumpercan be placed in the prosthetic system to prevent rotation of theprosthetic components around a superior-inferior axis until the bumperis resorbed by the body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an anterior perspective view of an embodiment of theinvention showing the prosthetic system in extension.

FIG. 1B is an anterior perspective view of the embodiment shown in FIG.1A, showing the prosthetic system in flexion.

FIG. 2 is a posterior view of an embodiment of the prosthesis.

FIG. 3 is an anterior view of an embodiment of the prosthesis.

FIG. 4 is a top view of an embodiment of the prosthesis.

FIG. 5 is a posterior exploded view of an embodiment of the invention.

FIGS. 6A-E are side views of an embodiment of the prosthesis progressingfrom extension in FIG. 6A to flexion in FIG. 6E.

DETAILED DESCRIPTION

FIG. 1 illustrates a perspective view of an embodiment of a prostheticsystem in extension in FIG. 1A and at 90° flexion in FIG. 1B. FIGS. 2and 3 show a posterior view and an anterior view, respectively, of anembodiment of the prosthetic system in extension. FIG. 4 shows a topview of an embodiment of the prosthetic system in extension. While theillustrated embodiment is a knee joint, the present invention could beused in other joints, such as a hip joint or a shoulder joint. Theprosthetic system includes a femoral component 100, a tibial component200, a tibial insert 300, and a mechanical linkage component or hingeportion 400. In surgery with the prosthetic system, the tibia and femurare recessed with the intramedullary canals of the tibia and the femursurgically prepared to receive stems. The present invention requires thesame bone cuts and instrumentation as a primary or revision system, suchas, for example, the Genesis II total knee system from Smith & Nephew.Only one additional cut is necessary with the present invention.

The tibial component 200 includes a tibial plateau 202 and a tibial stemportion 204. The tibial stem portion 204 includes a generallycylindrical portion 206 integrally formed with distal end 208 whichcomprises a Morse taper. The distal end 208 can have a long stemattached (not shown) via the Morse taper in a manner well known to thoseskilled in the art. Distal end 208 is fitted into the intramedullarycanal of the resected tibia, either with or without a long stem attachedto the Morse taper.

The femoral component 100 includes a pair of condyles 102, which aremetallic and highly polished and formed on a base portion 103 of thefemoral component 100. The condyles 102 engage with the tibial insert300. The femoral component 100 has a J-shaped cross section and as aresult has J-shaped condylar surfaces 102. These J-shaped surfaces haveat least two different radii of curvature: a distal radius and aposterior radius. In the preferred embodiment shown, the distal radiusof curvature is larger than the posterior radius of curvature. The baseportion 103 includes a pair of posteriorly extending lobes 104 thatconnect to the hinge element 400 as described below. Integral with andpositioned between the condylar portions is a central housing 106 havinga top wall 108 and side walls 110. Angularly mounted from the top wall108 is a femoral stem portion 112 having a proximal end 114 comprising aMorse taper. The proximal end 114 can have a long stem attached (notshown) via the Morse taper in a manner well known to those skilled inthe art, or can be used without a long stem. The proximal end 114 of thestem portion 112 is inserted into the intramedullary canal of theresected femur, either with or without a long stem attached to the Morsetaper.

For simplicity, the preferred embodiment is described as having tworadii of curvature along the distal and posterior surfaces of femoralcomponent 100, with the distal radius being larger than the posteriorradius, as discussed above. It is to be understood that it may beadvantageous to incorporate one or more additional radii of curvaturealong the outer surface of the femoral component. In particular, a thirdradius may be used to form the curvature at the proximal surface oflobes 104 of the posterior condyles. The number and relationship of theradii of curvature may be varied without departing from the spirit andscope of this invention.

FIG. 5 is an exploded posterior perspective view of an embodiment of theprosthetic system. A post 210 is positioned on the top side of thetibial plateau 202 in the vertical (superior-inferior) directionperpendicular to the tibial plateau 202 and a stop 212 is positioned atthe posterior portion of the tibial plateau 202. The post 210 receives acap 414. The cap 414 is secured to the post 210 via a fastener 420 orany other method known to those skilled in the art.

The tibial insert 300 has a top bearing surface 302 and a recessedportion 304 with an aperture 306 and a notch 308. The tibial insertaperture 306 receives the tibial post 210 and cap 414 so that the tibialinsert 300 is situated on the tibial plateau 202. In the illustratedembodiment, the tibial insert rotates about the vertical(superior-inferior) axis. The rotation of the tibial insert iscontrolled by the notch 308 at the posterior of the tibial insert 300and the stop 212 of the tibial plateau 202. The condyles of the femoralcomponent translate on the bearing surface 302 of the tibial insert 300.

The hinge portion 400 includes a link 402 with an aperture 404 in themedial-lateral direction on the posterior end and an aperture 406 in thesuperior-inferior direction on the anterior end. Two bushings 408 fitinto the ends of the posterior aperture 404. The posterior end of thelink 402 is positioned between the lobes 104 of the femoral component100. The link 402 is rotatably connected to the femoral component by anaxle 410 positioned in apertures 105 of both lobes 104 of the femoralcomponent 100 and through the posterior aperture 404 and bushings 408 ofthe link. The axle 410 is secured to the lobes 104 via two axle clips412 on each end of the axle 410. Alternatively, the axle 410 could besecured to the lobes 104 via any method known to those skilled in theart. The anterior aperture 406 of the link receives the tibial post 210and cap 414. The link 402 translates up and down the post 210 and cap414 and rotates about the superior-inferior axis. The cap 414 has asheath portion 418 and a lip portion 416 that is positioned at itsproximal end. Alternatively, the cap 414 could fit only on the top ofthe tibial post 210 and not have a sheath portion. The anterior aperture406 of the link 402 has a smaller diameter in its distal half so thatthe lip 416 of the cap 414 catches on the smaller diameter and controlsthe translation of the link 402.

The cap 414 is a separate piece and different sizes are available tocorrespond with the thickness of the tibial insert 300. This allows thefemoral component to be pre-assembled and allows the surgeon to selectthe appropriate cap size and tibial insert size during surgery to allowfor proper operation of the prosthetic system. This differs from mostsystems where the surgeon must assemble the femoral component based onthe tibial insert size.

As demonstrated by FIG. 6, the operation of the hinge portion 400 allowsfemoral roll-back and controls this movement. FIG. 6 shows theprosthetic system moving from full extension (0° flexion) in FIG. 6A to140° flexion in FIG. 6E. The prosthetic system is in 45° flexion in FIG.6B, 90° flexion in FIG. 6C, and 120° flexion in FIG. 6D. As shown inFIG. 6A, when the knee is in extension the link 402 is towards the topof the tibial post 210 and cap 414. Distraction is prevented by the cap414 of the tibial post 210 catching on the inner diameter of theanterior link aperture 406. As the knee moves from extension to flexionthe link 402 moves down the tibial post 210 and cap 414. The link 402does not move in the anterior-posterior directions, which allows theaxle 410 to move only along the superior-inferior axis.

As shown in FIG. 6, the axle moves in the inferior direction when theknee moves from extension to flexion and moves in the superior directionwhen the knee moves from flexion to extension. Axle 410 moves inferiorlyuntil posterior movement of the femoral component 100 positions femoralcomponent 100 so that it rides up the posterior lip of tibial insert300, then axle 410 moves superiorly. In the embodiment shown, thistranslates into inferior movement of axle 410 when the knee move from 0°to about 90° flexion, no vertical movement of axle 410 from about 90° toabout 120°, and superior movement from about 120° to about 140° flexion.The exact motion of axle 410 through the range of knee flexion maychange depending on the size of the components and other design featuresnot critical to this invention.

The center of contact of the condyles 102 on the bearing surface 302moves in the posterior direction as the knee moves from extension toflexion and moves in the anterior direction when the knee moves fromflexion to extension. In FIG. 6, P represents the center of contact ofthe condyles and the bearing surface. Because the radius of curvature ofthe condyles decreases when the knee moves from extension to flexion,the distance X from the center of contact P to the center point of theaxle decreases from extension to flexion, up to about 120° of flexion inthe embodiment shown.

The freedom of the axle 410 to move superiorly-inferiorly while linkingthe femoral and tibial components via the link 402 and the offset ofaxle 410 relative to the center of rotation of the femoral component 100result in roll-back of the femoral component during flexion whilemaintaining contact between the femoral component and the tibial insert300. Increasing the posterior offset of the axle 410 from the center ofrotation of the femoral component 100 causes an increasing anteriorshift in the center of contact P between the femoral component 100 andthe tibial insert 300 in extension and increasing travel of the link 402down the post 210 and cap 414 in flexion. Increasing inferior offset ofthe axle 410 from the center of rotation of the femoral component 100causes an increasing posterior shift in the center of contact betweenthe femoral component and the tibial insert in flexion and decreasingtravel of the link 402 up the post 210 and cap 414 in extension.

The ability of the link 402 to travel superiorly-inferiorly on thetibial post 210 and cap 414 allows specific combinations of link length,anterior-posterior offset and superior-inferior offset of the axle 410so that the anterior-posterior location of the center of contact betweenthe femoral component and the tibial insert as a function of flexion canbe specified and controlled. For example, less roll-back may bedesirable for smaller sized knees. This motion is further tailored bycombining the above described movement with the two different radii ofcurvature (larger distally and smaller posteriorly) in the condylesection of the femoral component—illustrated by decrease of the distanceX as the knee moves from extension to flexion in FIG. 6. Depending onthe specific objectives regarding the occurrence of roll-back duringflexion, a hinge knee could be designed according to this inventionhaving a single radius of curvature on the femoral component, or two ormore radii of curvature. While two radii are shown in the preferredembodiment, it is to be understood that the principles of this inventionare not to be so limited.

In specifying the motion as a function of flexion (kinematics), theperformance of muscle and other soft tissues can be optimized. Forexample, femoral roll-back is recognized as improving efficiency of theextensor mechanism. In general, roll-back is a posterior shift in thecenter of contact of the femoral component on the tibial component asthe knee flexes and an anterior shift in the center of contact of thefemoral component on the tibial component as the knee extends. Threeparameters define and control kinematics, including roll-back, in theprosthetic system of the current invention. The first parameter is theanterior-posterior and superior-inferior placement of the axle in thefemoral condyles. With the first parameter, the axis of rotation ispositioned in the posterior portion of the femoral component, withoutimposing undue structural load away from the natural load axis of theknee bone structure. This way the load axis is not skewed in theanterior-posterior or medial-lateral direction from natural load axis oftibia. The second parameter is the two different radii of curvature inthe J-curve section of the femoral component. The third parameter is thelength of the link. Tailoring these parameters according to the implanttype and size optimizes the kinematics and joint performance and allowscontrol of roll-back.

The prosthetic system additionally does not allow subluxation in themedial-lateral directions or in the anterior-posterior directions,because the tibial component is mechanically linked to the femoralcomponent.

The prosthetic system according to one embodiment shares common designelements of a primary and revision system, such as the Genesis II fromSmith & Nephew or other total knee system. This allows a surgeon tointra-operatively convert from a primary or revision implant to a hingedimplant with common bone cuts and the same instrumentation rather thanutilizing a separate system and instruments. The prosthetic systemaccording to one embodiment requires only three additional cuts than arerequired in a typical revision knee replacement procedure. The extracuts are needed to accommodate the wider central housing of the femoralcomponent used in the present invention, change the tibial plateau to aneutral (0°) slope, and accommodate the axle. Even with these fewadditional cuts, the system according to this invention providesrelatively simple intra-operative conversion from a standard revisionknee to a hinged knee. The prosthetic system according to one embodimentutilizes a pre-assembled femoral component, so that the surgeon does nothave to assemble a femoral component based on the tibial insert. Thetibial plateau of the present invention can accommodate severalthicknesses of tibial inserts enabling the surgeon to choose a tibialinsert of appropriate thickness and corresponding cap and use apre-assembled femoral component. Allowing conversion to the hinge kneeof the present invention intra-operatively reduces the risk to thepatient by reducing the procedure time.

The prosthetic system according to one embodiment is designed to acceptbody segments to replace the entire bone in the area of the knee (femuror tibia) in the case of tumor resections or trauma. Such body segmentsmay be secured to the Morse tapers on stem portions 204 and/or 112 in agenerally conventional manner or by any other attachment means known inthe art. If an additional prosthesis is required for replacement ofbone, it is provided as a separate component.

In general, an implant is unstable for several weeks after surgerybecause there is no scar tissue in the joint envelope. During this time,which typically lasts approximately six weeks but can vary considerablyfrom one patient to the next, it is desirable to not allow rotationalong the superior-inferior axis. In an embodiment of the prostheticsystem, a bioresorbable bumper 600 (shown in FIG. 5) is placed in thegap of the tibial insert 300 in between the notch 308 and the stop 212of tibial plateau. This allows an implant to be fixed when implanted,but later allows the implant to rotate as the material is resorbed bythe body. The rate of resorption can be selected by choosing the correctcomposition for bumper 600 to meet the particular needs of the patient.

One embodiment according to this invention is a prosthetic system andkit of parts for replacement of joints, such as a knee. Along with thecomponents described above, the kit of parts includes cutting blocks,reamers, and trials.

One method of using the prosthetic system according to this inventionfor replacing a joint, such as a knee, is as follows:

(1) resect the proximal end of the tibia to expose the tibialintramedullary canal of the tibia;

(2) resect the distal end of the femur to expose the femoralintramedullary canal;

(3) connect the tibial stem and the femoral stem to the prostheticsystem;

(4) insert a femoral stem into the femoral intramedullary canal; and

(5) insert a tibial stem into the tibial intramedullary canal.

-   This method additionally includes selecting the appropriate tibial    insert and cap intra-operatively.

In an alternative embodiment, the mechanical linkage component can beused in other joints allowing the axis of rotation of the joint totranslate in order to provide controlled roll-back and naturalkinematics during flexion or extension of the joint.

The disclosure of systems and processes as recited above is not intendedto limit the scope of the present invention. Various linking mechanismscan be used that allow the center of contact between the condyles andthe tibial insert to move posteriorly during flexion, provide for themovement of the axis of extension-flexion rotation in thesuperior-inferior direction, allow and control rotation about thesuperior-inferior axis, and offset the axis of rotation from thesuperior-inferior axis in order to provide the natural kinematics of theknee joint or other joint.

What is claimed is:
 1. A prosthetic system for performing the functionof a knee joint, comprising: (a) a tibial component having a tibialplateau that has a top side and a bottom side; (b) a post having aproximal end and a distal end, the post adapted to project from the topside of the tibial plateau; (c) a tibial insert having an aperture, thetibial insert adapted to be positioned on the top side of the tibialplateau with the post received in the aperture; (d) a femoral componenthaving a pair of condyles; and (e) a mechanical linkage that includes anaxle adapted to extend between the condyles, the mechanical linkagebeing adapted to connect the femoral component and tibial component andto maintain the axle at a fixed distance from the post while centers ofcontact between the condyles and the tibial insert move posteriorlyalong the tibial insert during flexion.
 2. The prosthetic system ofclaim 1, wherein the tibial insert has a posterior notch and the topside of the tibial plateau has a posterior stop, whereby the tibialinsert's rotation is limited by the posterior stop.
 3. The prostheticsystem of claim 2, further comprising a bioresorbable bumper adapted tofit in the notch and surround the stop to prevent rotation of the tibialinsert.
 4. The prosthetic system of claim 1, wherein a first distancebetween the center point of the axle and the distal portion of thecondyles is different than a second distance between the center point ofthe axle and the posterior portion of the condyles.
 5. The prostheticsystem of claim 1, wherein the mechanical linkage further comprises alink having an anterior end and a posterior end, the link adapted to beconnected to the axle at the posterior link end and to engage the postat the anterior link end.
 6. The prosthetic system of claim 5, furthercomprising an anterior aperture in the link and a cap adapted to mounton the post, the cap having a lip portion that extends beyond thediameter of the proximal end of the post, the anterior link aperturehaving a smaller diameter at its distal end, whereby the lip portionengages the smaller diameter of the anterior link aperture and preventsdistraction of the prosthetic system.
 7. The prosthetic system of claim6, further comprising a cap retaining fastener for fastening the cap tothe post.
 8. The prosthetic system of claim 5, further comprising aposterior aperture in the link and two bushings adapted to fit in theposterior link aperture and receive the axle.
 9. The prosthetic systemof claim 5, wherein the link links the tibial component with the femoralcomponent and with the tibial insert in between the tibial component andthe femoral component so that there is a center of contact between thecondyles and a bearing surface of the tibial insert, the link adapted tomove in the superior-inferior directions and restrained from movement inthe anterior-posterior directions, wherein the center of contact betweenthe condyles and bearing surface moves in the anterior-posteriordirection as the femoral component moves through extension and flexion.10. The prosthetic system of claim 9, wherein the link and tibial insertare adapted to rotate about an inferior-superior axis.
 11. Theprosthetic system of claim 5, wherein the post is configured to be fixedto the tibial component, and the link is configured to travel superiorlyand inferiorly along the post.
 12. The prosthetic system of claim 11,wherein the mechanical linkage is configured to constrain superiormovement of the axle during flexion and extension.
 13. The prostheticsystem of claim 5, wherein the link has an anterior aperture adapted toreceive the axle, and the link has a posterior aperture configured toreceive the post.
 14. The prosthetic system of claim 13, wherein theanterior aperture is defined along a superior-inferior axis, and theposterior aperture is defined along a medial-lateral axis.
 15. Theprosthetic system of claim 5, wherein the link has an anterior portionthat extends around the post, the anterior portion having a superioredge, and wherein the axle has a central axis located inferior to thesuperior edge of the anterior portion of the link.
 16. The prostheticsystem of claim 1, wherein the mechanical linkage is configured topermit superior-inferior movement of the axle during flexion andextension.
 17. A knee prosthesis comprising: a tibial componentincluding a tibial plateau having a top side and a bottom side; a posthaving a proximal end and a distal end, the post adapted to project fromthe top side of the tibial plateau; a tibial insert having an aperture,the tibial insert adapted to be positioned on the top side of the tibialplateau with the tibial insert aperture adapted to receive the post; afemoral component having a pair of condyles; and a mechanical linkagefor connecting the tibial component and the femoral component, whereinthe mechanical linkage defines a flexion-extension axis about which thefemoral component can rotate with respect to the tibial component;wherein, when the mechanical linkage connects the tibial component andthe femoral component, the flexion/extension axis is positioned at adistance from the post, and centers of contact between the condyles andthe tibial insert move posteriorly along the tibial insert duringflexion while the mechanical linkage maintains the distance of theflexion-extension axis from the post.
 18. The knee prosthesis of claim17, wherein the mechanical linkage permits superior or inferior movementof the flexion/extension axis along the post during flexion andextension.